The death and destruction associated with armed conflict often does not end with the conclusion of a peace treaty between warring parties. Rather, the remnant of armed conflict—land mines—remains a destructive threat long after hostilities have ended. In this regard, at present land mines are known to have caused substantial humanitarian and economic harm in regions throughout the globe, particularly in those regions which no longer host armed conflict. International experts estimate that up to one-hundred and ten million land mines remain to be cleared and that more than five-hundred civilians are killed or maimed every week by land mines. Many of these victims are children.
Despite the enormity of the problem of land mine clearance, such is known to be an extraordinarily challenging task. At the current rate at which land mines are cleared from their deadly, stealth positions, it has been estimated that more than one thousand years will be required to remove all of the land mines which have already been positioned, not accounting for those land mines which are continually placed elsewhere in the world. Nevertheless, for decades and at present, the most common method of detecting land mines remains the garden-variety metal detector. However, the usefulness of the metal detector falls off sharply in the face of a land mine having little or no metallic content, for instance a buried plastic land mine.
Ground penetrating radar (GPR) has been developed precisely for the purpose of detecting buried targets not based upon the metallic contents thereof, but based upon differences in the dielectric permittivity of the buried targets as compared to the surrounding environs. Accordingly, GPR has been recognized as a viable alternative to metal detection in the quest for a viable plastic land mine detector. Currently, two types of GPR systems have been investigated for their respective effectiveness in the detection of buried plastic land mines. One type of GPR system, the downward-looking GPR system, has shown promise in its ability to detect land mines, albeit downward-looking GPR systems suffer from some inherent limitations. The other type of GPR system, the forward looking GPR system, can be more complex than the downward-looking GPR system, albeit the inherent limitations of downward-looking GPR systems are not present.
In the downward-looking GPR system, antennae can be placed near the surface of the earth from whence radar signals can be transmitted downwardly and reflected back to the antennae. Consequently, strong radar signals can be reflected directly from the ground surface back into the antennae. Removing these strongly reflected signals, referred to as “ground bounce”, can be a challenging problem although conventional downward-looking GPR systems have been effective in removing substantial portions of the ground bounce signal from the radar imagery. Still, the utility of the downward-looking GPR system can be defeated through the time consuming nature of the operation of the downward-looking GPR system when interrogating a large area. Additionally, the short standoff distance between a system and mines can be problematic and dangerous when clearing a mine field. Accordingly, a forward looking GPR system can be desirable in many circumstances.
In a forward-looking GPR system, the antennae can be mounted on the front of a vehicle configured for deployment in a mine field. The forward-looking GPR system can capture radar signals at equally spaced positions as the vehicle moves forward in the mine field. Synthetic aperture radar (SAR) images can then be formed from the received signals. In this way, ground bounce can be reduced and a large standoff distance can be established between the vehicle and the mines in the mine field. Still, several problems remain in the use of a forward-looking GPR system which can defeat its effectiveness as a land mine clearance tool.
For instance, as the antennae of the forward-looking GPR system transmit energy at an acute angle to the ground and any land mines buried there beneath, most of the radar transmitted energies are reflected off the targets away from the antennae and only a very small fraction of the transmitted energy can be received by the antennae. Additionally, deeper buried mines can produce even weaker reflected signals. To compound matters, the nearly identical dielectric coefficients of plastic land mines and the surrounding soil can produce weak recognition as plastic mines cannot be seen convincingly in the spatial domain in the presence of clutter. Hence, detecting buried plastic mines can be extraordinarily challenging for a forward-looking GPR system.
In the past few years, several methods have been proposed for the forward-looking GPR systems and other similar detection systems. In particular, some signal processing techniques have been applied to radar signals produced from forward-looking GPR systems. In this regard, the effectiveness of several statistical signal processing techniques have been investigated for different mine types, burial depths and mine placements, including polarimetric whitening filter (PWF) and the generalized likelihood ratio test (GLRT). While the foregoing methods are reported to have been effective for detecting metal and surface mines with a high confidence, the same cannot be said of buried plastic land mines.
The failure of modern signal processing techniques to prove effective in detecting plastic land mines can be explained by reference to the failure of such techniques to fully utilize rich target signature information. More specifically, the conventional assumption that background signals can be modeled as Gaussian, log normal, or other such typical distribution can be inappropriate in the context of plastic land mine detection. In effect, the resultant constant false alarm rate (CFAR) detectors can be considered merely as energy based detectors.
Conventionally, the land mine detection problem had been formulated as to detect a target signal corrupted by several interference signals. For the sake of mathematical tractability, interference signals usually can be modeled as an additive white Gaussian noise (AWGN). In this respect, a matched filter has proven to be optimal under the AWGN assumption. Still, in practical applications, the Gaussian assumption of the interference signals has proven merely to be approximate rather than exact. In the buried plastic land mine detection context, the interference term actually has been shown to include two distinct portions: a measurement noise which may be properly modeled as AWGN; and a signal reflected from the ground or clutter which is non-stationary in nature and substantially non-Gaussian due to the time-varying nature of the environment.
The design of a land mine detector can be difficult when the statistics of the interference signals are not known precisely. Specifically, when the interference signal is nonstationary in nature, the detector must incorporate a time-varying structure, making the detector design even more difficult than ordinary. Moreover, when designing a detector, experts recommend the use of a priori information for the target signals where possible, such as the signal waveform or signature. The a priori information typically can be determined from a set of given training signals. To that end, one may estimate the signal waveform by simply averaging the training signals. In this case, the estimated target signal will converge to the true signal asymptotically.
Nevertheless, under the condition that the number of the training signals is limited and there exists a strong nonstationary interference, the averaging method may not be the best choice. To make matters worse, there may exist time-shift and frequency-shift issues that must be taken into account. Indeed the target signal can be so weak and embedded within a noisy background such that one cannot estimate its starting time correctly from the training signals. Moreover, due to the complicated operating environment, the frequency-shift may occur for the training signals. Accordingly, conventional techniques for processing signals from forward-looking GPR systems have not been effective for detecting most mines, particularly buried plastic land mines.